1. Field of the Invention
This invention relates to potentiometric measurement of ion activity or concentration in a solution. Specifically, it relates to a multiple membrane electrode device which amplifies the potential directed to measuring apparatus therefor by virtue of the fact that the potential sensing electrodes and reference elements associated with each membrane are connected in electrical series relationship to the apparatus.
2. Summary of Prior pH Glass Electrode Practices and Theories
The potentiometric measurement of the cationic and anionic activity in a solution has long been conducted using ion selective electrodes and a reference electrode which are both immersed in the solution to be tested and electrically connected to a potential measuring device such a pH meter or potentiometer.
In the case of pH measurements, the sensing or indicating electrode usually is made up of a non-conducting glass tube called the body which has a bulb sealed thereto made up of special conductive glass generally called the pH sensing membrane. The glass body is filled with a buffered electrolyte whose pH value and ionic concentration are fixed. An internal reference member is immersed in the buffered electrolyte solution. Typically, the reference member may be Ag/AgCl. This configuration assures that constant potentials are developed on the inner surface of the glass membrane and on the internal reference member. Thus, when the electrode is immersed in a solution of pH 7, the sum of these fixed voltages approximately balances the voltage developed on the outer surface of the glass membrane and a separate reference element. Under these conditions, the total potential output of the system is very nearly 0 mV.
The reference element serves to complete the electrical measuring circuit. A simple electrically conductive wire immersed in the sample solution will satisfy this purpose. However, in actual practice, the use of a conductive wire is susceptible to voltage changes, dependent on the time and solution and the sample composition. In order to avoid this problem, it is the general practice to use a reference element, e.g., calomel or Ag/AgCl immersed in an electrolyte filling solution of fixed ionic concentration contained in the probe body. This produces the required constant voltage, no matter what the sample composition. The electrical circuit is completed by allowing a small flow of the electrolyte to pass through a porous junction in the probe tip.
A combination pH measuring electrode containing both a pH sensing electrode and a reference element combined in a single probe body have also long been available. The annular space surrounding the inner tube contains the reference element and electrolyte. Solution contact is made through a porous junction in the outer wall. The combination electrode has the advantage of convenience in field use and the ability to take measurements in small sample vessels where an electrode pair would not fit.
Since the early 1900's, when the first observations were made of the potentiometric relationship of a glass membrane with a pH solution, various attempts have been made to explain the glass membrane potential. Although the pH glass electrode is one of the most widely used analytical tools, it is believed to be one of the least understood. Prior investigators have attempted to explain that the potential of a thin glass film is attributed to the selective permeability of mobility of the H.sup.+ ion across the glass-aqueous solution interface. The concept of an actual penetration through the glass membrane by hydrogen ions was first disproved in the 1940's and reaffirmed in the 1960's. Notwithstanding this clear evidence to the contrary, some investigators still incorrectly cling to the theory that the glass membrane is permeable to the H.sup.+ ion.
The now disproved adsorption-potential theory postulated an adsorbed layer of hydrogen ions on the glass surface causing a potential drop at the glass-solution interface, corresponding to the difference in chemical potential between the free and adsorbed ions. It was suggested that the gel layer of the glass membrane acted as an ion exchanger producing a phase boundary potential at which the H.sup.+ ion exchanged with the Na.sup.+ ion. Experimental results do not support this ambiguous proposal. If such an exchange did occur, there would be no net change in interfacial charges. Furthermore, the sodium ion in the gel layer of the membrane would eventually be depleted after long usage resulting in a failure of the glass electrode. However, it is well established that pH glass electrodes have a long useful life. It is also known that a quartz glass membrane containing no sodium is useful as a pH glass electrode.
It appears that prior investigators devoted an inordinate amount of time to interpreting the phenomena of glass electrodes for pH measurement purposes in thermodynamic terms. Nernst equations, which in simplified form may be expressed as ##EQU1## where E equals the measured voltage; E.sub.o is the total of all constant voltages in the measuring system; R is the Gas Law constant; T is the temperature in .degree. K; and F is Faraday's constant. However, more recent investigators have pointed out that in using this mathematical relationship, electrochemists have in effect tried to do the impractical, i.e., treat highly thermodynamically irreversible electrode reactions by a series of approximations and perceived misconceptions on the basis of reversible thermodynamics. Quantitative and instrumental analyses have inaccurately treated the glass electrode as a battery obeying the principles of reversible thermodynamics and the Nernst equations.
Similarly, ionic activity other than H.sup.+ and OH.sup.- in solutions has long been analyzed using an ion selective sensing electrode and a reference element immersed in the solution. For example, in order to determine the cations and anions present in a solution involving metal ions, the sensing electrode is generally of the same cation as that to be sensed. If the ionic activity of copper in a copper solution is to be measured, then a copper sensing electrode is utilized with an appropriate reference element which may be a simple electrically conductive wire as previously indicated.
3. Summary of New Theory
A battery is a device containing no insulator and producing an electric current through chemical redox reactions that occur in cells that are placed in series. Each cell contains an anode and a cathode that are immersed in an electrolyte medium. Connecting the anode and the cathode to the external circuit causes an electric current flow until chemical reactions cease. Mostly it is reversible, following the principles of equilibrium thermodynamics, i.e., the Nernst equation. On the other hand, a capacitor is a device for storing electric charges through two conducting plates between which there is a dielectric in which no reversible redox reaction takes place. By connecting two charging plates, no significant current flows. For a capacitor, C =q/V, where C is the capacitance, q is the charge and V is the potential difference.
An ion sensing electrode may be generally represented in a cell as follows wherein the cation in a solution is represented as M.sup.+ : ##EQU2## It has been recognized for a long time that there are no redox reactions involved in the potential development of glass electrode. Based on the definitions of a battery and a capacitor, it is not accurate to say that the electrode component of a pH measuring system is comparable to a battery where voltage changes with pH. Instead, it is believed that a potential sensing electrode in conjunction with a reference element is comparable to a capacitor rather than to a battery.
4. Description of the Prior Art
Measurement of the ionic potential test solutions with a plurality of test units has previously been suggested, but the electrodes have not been placed in electrical series relationship to function as capacitors which amplify the signal such that the total potential is equal to the sum of each individual membrane electrode potential.
U.S. Pat. No. 4,155,814 issued to Tejfalussy, et al. on May 22, 1979 describes an electrode system consisting of a number of working electrodes as well as a series of counterelectrodes and reference electrodes in series. In one embodiment, a single extended electrode membrane is provided with a number of individual reference electrodes in electrical parallel relationship. This device permits measurements simultaneously at a number of points equal to the number of reference electrodes. However, the result is simply to permit measurement of pH over a relatively wide area and there is no amplification of the signal.
In Dahms U.S. Pat. No. 3,556,950 issued Jan. 19, 1971, a plurality of electrodes each sensitive to a different dissolved gas or ion and a single reference electrode are connected in series so that a sequential measurement may be made of the different gas or ion to be analyzed. In Robinson U.S. Pat. No. 3,248,309 issued Jan. 26, 1966 describes an automatic titrator wherein continuous analysis may be carried out on very small quantities of the solution to be tested. In U.S. Pat. No. 870,674 of Nov. 12, 1907 in the name of Guess, et al., samples to be tested electrochemically may be analyzed in a predetermined sequential series.
Other patents disclosing multiple electrodes include Schwab, U.S. Pat. No. 3,787,307 of Jan. 22, 1974; Boeke, U.S. Pat. No. 4,133,732 of Jan. 9, 1979; Mattson, U.S. Pat. No. 4,404,065 of Sept. 13, 1983; Vematsu, U.S. Pat. No. 4,519,890 of May 28, 1985; Watanabe, U.S. Pat. No. 4,647,362 of Mar. 3, 1987; and Jackle, U.S. Pat. No. 4,686,011 of Aug. 11, 1987.